1. Field of the Invention
The present invention is directed to polyisocyanates which contain allophanate groups and isocyanurate groups and have a low solution viscosity and may be formulated to provide coatings with improved dry times and good environmental etch resistance. The present invention is also directed to a process for the production of these polyisocyanates from a mixture of diisocyanates and to their use in two-component coating compositions.
2. Description of the Prior Art
Polyisocyanates containing isocyanurate groups are known and disclosed in U.S. Pat. Nos. 3,487,080, 3,996,223, 4,324,879 and 4,412,073. While these polyisocyanates possess many outstanding properties, they do require improvement. First, the viscosity of commercially available polyisocyanates containing isocyanurate groups needs to be reduced in order to reduce the amount of solvent which is necessary to obtain a suitable processing viscosity. Presently, there are an increasing number of government regulations which limit the amount of volatile solvents which may be present in coating systems. Therefore, isocyanurate group-containing polyisocyanates may be precluded from certain applications because it is not possible to reduce the viscosity of these polyisocyanates to a suitable processing viscosity without using high amounts of solvent.
Second, coating compositions containing the isocyanurate group-containing polyisocyanates must have fast dry times and result in coatings with good environmental etch resistance in order to withstand the effects of acid rain.
It has been proposed in U.S. Pat. No. 4,801,663 to reduce the viscosity of isocyanurate group-containing polyisocyanates prepared from 1,6-hexamethylene diisocyanate (HDI). By terminating the reaction at a very low degree of trimerization higher contents of the monoisocyanurate of HDI are obtained and the quantity of polyisocyanates containing more than one isocyanurate ring is reduced. Because these latter polyisocyanates have a much higher viscosity than the monoisocyanurate, the resulting polyisocyanates have a reduced viscosity. However, a significant disadvantage of this system is that because the reaction is terminated at a very low degree of trimerization, the overall yield is very low and the amount of HDI which must be separated from the product is substantially increased. In other words the small reduction in viscosity is offset by a significant increase in the production cost of the product. Further, the resulting product does not possess optimum compatibility with highly branched polyester resins.
Accordingly, it is an object of the present invention to provide polyisocyanates which have a reduced solution viscosity and may be formulated to provide coatings with improved dry times and good environmental etch resistance, while possessing the desirable properties of known polyisocyanates containing isocyanurate groups. It is an additional object of the present invention to provide polyisocyanates which may be produced at reasonable production costs and which are obtained in high yields. Surprisingly, these objectives may be achieved in accordance with the present invention as described hereinafter by the incorporation of specific monoalcohols before or during the trimerization of a specific mixture of diisocyanate starting materials in order to produce a polyisocyanate containing isocyanurate and allophanate groups.
U.S. Pat. Nos. 4,582,888, 4,604,418, 4,647,623, 4,789,705 are directed the incorporation of various diols in order to improve the compatibility of the resulting polyisocyanates with certain solvents and co-reactants. While the use of diols may improve the compatibility of the polyisocyanates, the diols do not reduce the viscosity of the polyisocyanurates for a given yield.
Many of these references as well as those previously set forth disclose the use of monoalcohols or glycols as co-catalysts for the trimerization reaction. However, none of these references suggest the incorporation of allophanate groups to reduce the viscosity of polyisocyanates containing isocyanurate groups. Further, these references teach that the use of these cocatalysts should be kept to a minimum since the resulting urethane groups reduce the drying time of coatings prepared from the polyisocyanates. In particular, U.S. Pat. No. 4,582,888 cautions against the use of any amount of monoalcohol which is in excess of that needed to dissolve the catalyst.
Japanese Publication 61-151179 discloses the use of monoalcohols containing 6 to 9 carbon atoms as co-catalysts for trimerization catalysts which do not trimerize HDI in the absence of a co-catalyst.
It has been disclosed in copending applications, U.S. Ser. Nos. 07/644,174 U.S. Pat. No. 5,124,427, 07/733,549 abandoned and 07/733,566 abandoned, to conduct the trimerization of organic diisocyanates in the presence of monoalcohols having molecular weights of up to 2500 to incorporate allophanate groups into the final products and reduce their viscosity. Even though these applications disclose the use of any diisocyanates having (cyclo)aliphatically-bound isocyanate groups as starting materials, they do not recognize the combination of properties which may be obtained by using mixtures of diisocyanates as starting materials as described hereinafter.